WASHINGTON For the first 10 weeks of President Trumps administration, no adviser loomed larger in the public imagination than Stephen K. Bannon, the raw and rumpled former chairman of Breitbart News who considers himself a virulently anti-establishment revolutionary out to destroy the administrative state.

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But behind the scenes, White House officials said, the ideologist who enjoyed the presidents confidence became increasingly embattled as other advisers, including Mr. Trumps daughter and son-in-law, complained about setbacks on health care and immigration. Lately, Mr. Bannon has been conspicuously absent from some meetings. And now he has lost his seat at the national security table.

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In a move that was widely seen as a sign of changing fortunes, Mr. Trump removed Mr. Bannon, his chief strategist, from the National Security Councils cabinet-level principals committee on Wednesday. The shift was orchestrated by Lt. Gen. H. R. McMaster, Mr. Trumps national security adviser, who insisted on purging a political adviser from the Situation Room where decisions about war and peace are made.

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Mr. Bannon resisted the move, even threatening at one point to quit if it went forward, according to a White House official who, like others, insisted on anonymity to discuss internal deliberations. Mr. Bannons camp denied that he had threatened to resign and spent the day spreading the word that the shift was a natural evolution, not a signal of any diminution of his outsize influence.

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His allies said privately that Mr. Bannon had been put on the principals committee to keep an eye on Mr. Trumps first national security adviser, Michael T. Flynn, a retired three-star general who lasted just 24 days before being forced out for misleading Vice President Mike Pence and other White House officials about what he had discussed with Russias ambassador. With Mr. Flynn gone, these allies said, there was no need for Mr. Bannon to remain, but they noted that he had kept his security clearance.